For many surgical procedures the use of minimally-invasive surgical tools/instruments is widely accepted. Such surgical instruments include needles, endoscopes (a bundle of optical fibers, or a small camera encapsulated by a catheter), and catheters. These instruments may be used for procedures such as biopsies, endoscopic imaging, and cardiac catheterization surgeries. These procedures are performed, e.g., to diagnose tissues or organs suspected of being cancerous or otherwise in need of treatment, and for treating cardiac disease or malfunction.
Several techniques exist for monitoring the position of an instrument inside a human, including X-Ray fluoroscopy. Conventional X-Ray fluoroscopy is designed to reduce/minimize X-ray dosage, but long procedures can produce significant doses in the patient. Further, long-term exposure of personnel conducting X-Ray fluoroscopy procedures is a concern. Also, metal instruments produce a very strong signal in response to X-rays, saturating detectors and thus producing a “halo” around the instrument's image such that a tip of the instrument may be obscured.
Other techniques for monitoring instrument position include using ultrasound or electromagnetic radiation. Ultrasound is common because when operated at low power its energy is not hazardous and because soft tissues in the body that contain water are ideal for transmitting and reflecting sound waves. An ultrasound imager transmits sound waves to a specific area in the body and detects reflected signals. The imager forms a gray-scale image where the position of each feature is calculated from the delay in the reflected sound wave, and different intensities of reflected signals appear as different levels of gray in the image. A high impedance mismatch between the instrument and the surrounding tissues results in strong scattering of the sound waves. Another challenge with locating the tip of an instrument using ultrasound results from the two-dimensional scanning nature of the ultrasound imaging procedure. At any given time, the operator can view the cross section of the instrument in the plane “cut out” by the array. Consequently, only when the instrument is coplanar with the scanning motion can one identify its shape and locate its tip easily; at any other angle only the part of the instrument that is coplanar with the imaged plane can be identified. To improve the acoustic signature of instruments, passive and active acoustic reflectors or transducers may be attached to instruments as discussed in U.S. Pat. No. 4,431,006 and U.S. Pat. No. 4,697,595, respectively.
Still other techniques exist for monitoring instrument location inside of a subject. Ultrasound transducers can be integrated with instruments such as a catheter. A position sensor may be attached to a needle and convey position information to an imager. A low-power RF emitter may be attached to the tip of an instrument and RF sensors outside of the body may be used to detect the position of the emitter. Non-concentric coils may be attached to an instrument to produce signals in response to externally-applied magnetic fields to allow determination of position and orientation of an instrument attached to the coils. Further, ultrasound pulses can be produced outside of a subject, conveyed to the inside of the subject using acoustical fibers, and reflected signals collected and transmitted with these same fibers.